There are numerous methods and systems for detecting radiation. In one type of detector, photocathodes are used in conjunction with microchannel plates (MCPs) to detect low levels of electromagnetic radiation. Photocathodes emit electrons in response to exposure to photons. The electrons can then be accelerated by electrostatic fields toward a microchannel plate. A microchannel plate is typically manufactured from lead glass and has a multitude of channels, each one operable to produce cascades of secondary electrons in response to incident electrons. A receiving device then receives the secondary electrons and sends out a signal responsive to the electrons. Since the number of electrons emitted from the microchannel plate is much larger than the number of incident electrons, the signal produced by the device is stronger than it would have been without the microchannel plate.
One example of the use of a photocathode with a microchannel plate is an image intensifier tube. The image intensifier tube is used in night vision devices to amplify low light levels so that the user can see even in very dark conditions. In the image intensifier tube, a photocathode produces electrons in response to photons from an image. The electrons are then accelerated to the microchannel plate, which produces secondary emission electrons in response. The secondary emission electrons are received at a phosphor screen or, alternatively, a charge coupled device (CCD), thus producing a representation of the original image.
Another example of a device that uses a photocathode with a microchannel plate is a scintillation counter used to detect particles. High-energy particles pass through a scintillating material, thereby generating photons. Depending on the type of material used and the energy of the particles, these photons can be small in number. A photocathode in conjunction with a microchannel plate can be used to amplify the photon signal in similar fashion to an image intensifier tube. The detector can thus be used to detect faint particle signals and to transmit a signal to a device, e.g., a counter, that records the particle's presence.
A photocathode may include one or more layers of material deposited or grown on a surface of the photocathode to provide anti-reflection properties, filtering properties, electron transportability properties, and other suitable properties associated with the photocathode. After the layers have been deposited or grown on the surface of the photocathode, the surface of the photocathode generally requires polishing to reduce the layer to a predetermined thickness to provide the desired photocathode properties. The polishing process generally includes translating the photocathode across a polishing pad and/or polishing compound for a predetermined amount of time. Thus, the amount of material removal from the photocathode is a function of the abrasive characteristics of the polishing pad and/or chemical etching properties of the polishing compound, the amount of pressure applied to the photocathode during polishing, and the amount of time the photocathode is polished.
Various types of retaining fixtures may be used to hold the photocathode during the polishing process. An example retaining fixture may include a retainer having a seating area to hold the photocathode as the photocathode is translated across a polishing pad. A weight may be disposed above the retainer opposite the seating area to apply a downwardly directed force to the photocathode during the polishing process. The retainer and weight may also be placed within an outer housing such that forces applied to the outer housing during the polishing process do not affect the force applied to the photocathode.
However, prior systems and methods for manufacturing a photocathode suffer several disadvantages. For example, chemical properties of the polishing compound may cause degradation or oxidation of various components of the retaining fixture, thereby affecting the interaction between the retainer and the outer housing. As a result of component degradation or oxidation, forces applied to the outer housing during the polishing process may be transferred to the photocathode and affect the amount of material removal from the polishing surface of the photocathode.
For example, the amount of pressure applied to the outer housing from one operator to another may differ and cause varying amounts of material removal from different photocathodes, thereby resulting in inconsistent photocathode properties. Additionally, the location, direction, and amount of pressure applied to the outer housing by the operator during the polishing process may vary, thereby resulting in a nonuniform layer thickness across the polished surface of the photocathode.